I'm still trying to get a hang of the parallels between arrow notation and the semantics of the Arrow typeclasses defined in Haskell. In particular, this question seems to have a very canonical example of a small counter written with arrow notation:
counter :: ArrowCircuit a => a Bool Int
counter = proc reset -> do
rec output <- returnA -< if reset then 0 else next
next <- delay 0 -< output+1
returnA -< output
Can someone show me how to convert this back into Haskell2010 without arrow notation?
{- |
+---------+
>Bool>--------------> |
| >------------------>Int>
+---------+ | arr f |
/----> delay 0 >---> >---------\
| +---------+ | | |
| +---------+ |
| |
\--------------------------------------/
-}
counter' :: ArrowCircuit a => a Bool Int
counter' = loop $ second (delay 0) >>> arr f
where
f (reset, next) = let output = if reset then 0 else next
next' = output + 1
in (output, next')
The recursive rec part is implemented using loop. The inner part that converts reset to output using next (and producing new next value) is just a pure function with two inputs and two outputs.